riscv: implement user_access_begin() and families

Currently, when a function like strncpy_from_user() is called,
the userspace access protection is disabled and enabled
for every word read.

By implementing user_access_begin() and families, the protection
is disabled at the beginning of the copy and enabled at the end.

The __inttype macro is borrowed from x86 implementation.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Cyril Bur <cyrilbur@tenstorrent.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250410070526.3160847-3-cyrilbur@tenstorrent.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: xhackerustc

arch/riscv/include/asm/uaccess.h

@@ -61,6 +61,19 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, unsigne
 #define __disable_user_access()							\
 	__asm__ __volatile__ ("csrc sstatus, %0" : : "r" (SR_SUM) : "memory")
 
+/*
+ * This is the smallest unsigned integer type that can fit a value
+ * (up to 'long long')
+ */
+#define __inttype(x) __typeof__(		\
+	__typefits(x, char,			\
+	  __typefits(x, short,			\
+	    __typefits(x, int,			\
+	      __typefits(x, long, 0ULL)))))
+
+#define __typefits(x, type, not) \
+	__builtin_choose_expr(sizeof(x) <= sizeof(type), (unsigned type)0, not)
+
 /*
  * The exception table consists of pairs of addresses: the first is the
  * address of an instruction that is allowed to fault, and the second is
@@ -368,6 +381,69 @@ do {									\
 		goto err_label;						\
 } while (0)
 
+static __must_check __always_inline bool user_access_begin(const void __user *ptr, size_t len)
+{
+	if (unlikely(!access_ok(ptr, len)))
+		return 0;
+	__enable_user_access();
+	return 1;
+}
+#define user_access_begin user_access_begin
+#define user_access_end __disable_user_access
+
+static inline unsigned long user_access_save(void) { return 0UL; }
+static inline void user_access_restore(unsigned long enabled) { }
+
+/*
+ * We want the unsafe accessors to always be inlined and use
+ * the error labels - thus the macro games.
+ */
+#define unsafe_put_user(x, ptr, label)	do {				\
+	long __err = 0;							\
+	__put_user_nocheck(x, (ptr), __err);				\
+	if (__err)							\
+		goto label;						\
+} while (0)
+
+#define unsafe_get_user(x, ptr, label)	do {				\
+	long __err = 0;							\
+	__inttype(*(ptr)) __gu_val;					\
+	__get_user_nocheck(__gu_val, (ptr), __err);			\
+	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
+	if (__err)							\
+		goto label;						\
+} while (0)
+
+#define unsafe_copy_loop(dst, src, len, type, op, label)		\
+	while (len >= sizeof(type)) {					\
+		op(*(type *)(src), (type __user *)(dst), label);	\
+		dst += sizeof(type);					\
+		src += sizeof(type);					\
+		len -= sizeof(type);					\
+	}
+
+#define unsafe_copy_to_user(_dst, _src, _len, label)			\
+do {									\
+	char __user *__ucu_dst = (_dst);				\
+	const char *__ucu_src = (_src);					\
+	size_t __ucu_len = (_len);					\
+	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u64, unsafe_put_user, label);	\
+	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u32, unsafe_put_user, label);	\
+	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u16, unsafe_put_user, label);	\
+	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u8, unsafe_put_user, label);	\
+} while (0)
+
+#define unsafe_copy_from_user(_dst, _src, _len, label)			\
+do {									\
+	char *__ucu_dst = (_dst);					\
+	const char __user *__ucu_src = (_src);				\
+	size_t __ucu_len = (_len);					\
+	unsafe_copy_loop(__ucu_src, __ucu_dst, __ucu_len, u64, unsafe_get_user, label);	\
+	unsafe_copy_loop(__ucu_src, __ucu_dst, __ucu_len, u32, unsafe_get_user, label);	\
+	unsafe_copy_loop(__ucu_src, __ucu_dst, __ucu_len, u16, unsafe_get_user, label);	\
+	unsafe_copy_loop(__ucu_src, __ucu_dst, __ucu_len, u8, unsafe_get_user, label);	\
+} while (0)
+
 #else /* CONFIG_MMU */
 #include <asm-generic/uaccess.h>
 #endif /* CONFIG_MMU */